200% UI scale should look fine on a "200% density" (This stuff isn't exact, think of it as a fuzz factor) screen, that it doesn't look fine points to underlying issues with how Windows handles it. I know when I've tried experimenting with it in the past UI elements just looked "wrong" (Like they were being scaled incorrectly)

I don't expect that there is any such replies to this topic.
I use Windows 8 Pro with 200% on a rMPB it goes pretty good, however not everything is suitable.
Here are some examples:
Desktop + Origin (very small even if the dpi is set to 200%) http://d.pr/i/fVN6

I'm blind (figuratively) and I use a 24" display at 1920x1080. Always have had to scale up font size to 150%.

You have to make sure that "Windows XP Style Scaling" is checked or else many applications, like Chrome, look horrible. Other than that it's usually cross platform GUI kits like GTK that have trouble scaling and cause fonts to overflow or be misplaced. It's usually not a problem for me.

I just changed the DPI on my work computer to verify that the same massively annoying bug still exists in Windows 7 DPI scaling. If you change your DPI, the superbar won't ever hide behind an app that's full screen. You have to change the superbar to autohide to get it to go away. This is incredibly annoying on an HTPC that frequently has full screen apps that you don't want to superbar hanging out on. This is more annoying than any of the blurry third party issues. Plus, regardless of who is to blame, there are so many inconsistencies when using high DPI mode in Windows that it's just hard to seriously use. They need to push third parties to update their apps with all these higher resolution screens coming out. It'll be impossible to use an HTPC on a 4k screen without DPI scaling.

That's an app bug not a windows bug. MediaPortal used to do this, but now works properly in hgh dpi.

I don't expect that there is any such replies to this topic.I use Windows 8 Pro with 200% on a rMPB it goes pretty good, however not everything is suitable.Here are some examples:Desktop + Origin (very small even if the dpi is set to 200%) http://d.pr/i/fVN6

For now the only apps that I found partially adapted to Retina screens are Office 2013, VS 2012 (fuzzy icon) and the Metro interface.

Besides, I have another question how to make IE 10 or Chrome displays images x2 Apple's website!

I would put other screenshot later.

Thanks for the screenshots

Regarding the bolded bit, that's a bit tough. IE has always handled this stuff a bit strangely (You'll see similar issues on high-dpi WP devices), and Chrome should get support in a future release (Since WebKit already exposes it to content in a non-standard way). Firefox can handle this (Via standard methods, and the same JS extension WebKit uses), but it's half disabled by default due to issues.

Interesting that Steam has issues since that uses DirectWrite (and by extension, Direct2D, hopefully), and that supports resolution independence by default, so they've either broken it, or are using some strange rendering method that breaks it.

Edit: O, for the rest of the stuff, disabling XP style scaling should "help" (It'll just be pixel doubled then, so blurry but scaled properly)

Again, it's a problem with XBMC. they need to fix their code and add support for high DPI. MP used to have the same issue, which has since been fixed for the last 6-12 months, not sure.

Tell XBMC to fix their broken code, or change to a working properly coded MC solution. both MediaPortal and Plex have no problem with the taskbar in High DPI.

Although I don't doubt XBMC is buggy, this happens to me as well and I don't even use or have XBMC installed. This is triggered for me whenever I go to full screen mode for any video online. Including but not limited to Youtube.

I really never see this on my HTPC at 150% with XBMC or even on my desktop when I set it to 125%, 135%, 150%, and 200%. XBMC 12 RC3 always stayed on top of the bar.
I've also used N64 emulator and YouTube in IE without issue.